A variety of emissions, such as nitrogen oxides (e.g., NO and NO2), are emitted in exhaust gases of internal combustion engines. In order to decrease emissions from motor vehicles, emissions are regulated via use of exhaust system components, such as catalytic converters. Additionally, various gas sensors, including NOx sensors, are employed to detect the emissions in exhaust gases.
During operation, accurate measurement of NOx in the exhaust gases may depend on temperature control of a NOx sensor. U.S. Pat. No. 6,228,252 describes a method to correct NOx concentration measurement of a NOx sensor via temperature detection of the sensor. In the cited reference, temperature detection of the NOx sensor is implemented by measuring internal resistance of a gas concentration measurement cell, as internal resistance is temperature dependent. Depending on a measured offset of the NOx sensor temperature from a target temperature, the NOx concentration measurement may be adjusted. An offset from the target temperature of the sensor may occur, for example, due to a sudden change in the temperature of the exhaust gases or due to a sudden change in the flow rate of the exhaust gases. Thus, temperature of a NOx sensor may be detected and, in the event of an offset, the NOx measurement may be corrected without additional temperature sensing components.
However, measuring the internal temperature of the NOx sensor by measuring internal resistance of the gas concentration measurement cell requires a temporary cessation of measuring the NOx concentration. Specifically, the approach relies on applying a constant voltage to the terminals of the measurement cell or applying a constant current through the measurement cell for calculating the resistance, and thus the temperature, of the measurement cell. The NOx concentration is measured by detecting a current through the measurement cell that changes proportionally to the concentration of NOx. Given the different requirements for measuring temperature and for measuring NOx concentration, these measurements may be time-multiplexed and there may be times when the NOx sensor is not measuring the NOx concentration or not measuring sensor temperature. Furthermore, even if the temperature could somehow be measured and used during the transient condition, errors in the transient NOx readings may still be generated, for example due to slow responsiveness of temperature readings, system effects, temperatures differences between the cells, etc, for example. In other words, even with this described temperature correction, erroneous NOx readings during transient conditions may still be generated.
The inventors herein have recognized the above problems and have devised an approach to address them. Thus, in one example, a method which includes adjusting the NOx concentration based on transient engine exhaust conditions is disclosed. Various transient engine exhaust conditions may be considered, such as changes in exhaust gas flow rate, changes in exhaust gas temperature, and/or changes in exhaust gas O2 concentration. Further, the method may consider the rate of change of such parameters, or other such indicative values.
In one specific example, the method may include generating a correction value for a NOx concentration reading from the NOx sensor based on rate of change of exhaust gas flow rate, and correcting the NOx concentration reading using the correction value. In this manner, it is possible to more continuously monitor the NOx concentration during transient engine conditions
Further, note that in addition to the corrections based on the transient exhaust flow conditions, various additional corrections may also be used, such as based on NOx sensor temperature, exhaust temperature, etc.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.